1. Field of the Invention
The present invention is generally related to position sensors and, more specifically, to a method and apparatus for attaching a gear tooth sensor to an automobile engine in such a way that the distal end of the sensor is positioned at a precise location relative to a rotatable target.
2. Description of the Prior Art
Many different types of sensors are known to those skilled in the art and many varieties of gear tooth sensors are also well known. In operation, a gear tooth sensor, or proximity sensor, typically comprises a permanent magnet in association with a magnetically sensitive component such as a Hall effect element or a magnetoresistive element. Sensors of this type are constructed in such a way that a ferromagnetic component passing through a predefined detection zone will cause the magnetically sensitive component to provide a signal identifying this event. The ferromagnetic object can be the teeth of a gear, whether or not the gear is intended to operate in the traditional manner of a gear or merely to provide a plurality of teeth extending from a rotatable target, in order to facilitate the detection of the teeth for the purpose of sensing the angular position or velocity of the rotating target. Devices of this type find many applications in automobiles, including sensors which monitor the position of crank shafts and cam shafts and, in addition, sensors which are used in automatic braking systems. A problem which occurs when sensors of this type are installed relative to a rotatable target is that the gap between the distal end, or operative end, of the sensor and the target must be maintained with a relatively high degree of accuracy. Otherwise, the signal level provided by the magnetically sensitive component of the sensor will not be properly calibrated to the angular position of the target.
U.S. Pat. No. 4,677,378, which issued to Tokura et al on Jun. 30, 1987, discloses a displacement sensor which has a magnetically responsive member and a pair of piezoelectric elements. The sensor is provided with first and second piezoelectric elements, a pair of electrodes at one and the other ends of each of the first and second piezoelectric elements and a permanent magnet mechanically fixed to one end of the first piezoelectric element. In addition, it provides a small piece which is mechanically fixed to one end of the second piezoelectric element and has approximately the same weight as the permanent magnet. A base plate to which the other ends of the first and second piezoelectric elements are mechanically fixed is provided wherein the electrodes provided at the other end of each of the first and second piezoelectric elements are electrically connected. The electrodes provided at one end of each of the first and second piezoelectric elements are used as output terminals. In addition, a pair of small pieces made of a magnetic material and a nonmagnetic material can be used instead of the permanent magnet and small piece.
U.S. Pat. No. 5,121,289, which issued to Gagliardi on Jun. 9, 1992, describes an encapsulatable sensor assembly. The description in the Gagliardi patent shows the inner construction of a gear tooth sensor which comprises a permanent magnet and a magnetically sensitive component. The assembly includes an external housing with internal support components for retaining an active sensor element and associated electrical circuitry in a predetermined relationship while providing a plurality of internal sequentially interconnected cavities for facilitating a flow of an encapsulating material from an injection port to a vented overflow in order to assure a complete fill of the internal cavities to restrain the internal components. The sensor assembly provides a structure for a void free encapsulation and internal leadwire stress relief concurrently with location control of the internal component while also providing a combination which is easily adaptable to automatic assembly and encapsulation techniques.
U.S. Pat. No. 5,140,262, which issued to Stolfus on Aug. 18, 1992, discloses a gear tooth sensor with a centerline in nonintersecting relation with a center of rotation of a rotatable member. The off-axis gear tooth sensor is provided with a centerline of the sensor that is disposed along a line which is not coincident with the center of rotation of the rotatable member to be sensed. Instead, the centerline of the housing in which a Hall effect element is disposed is positioned at a perpendicular distance from the center of rotation of the rotatable member which is determine as the function of range of dimensions which define the allowable gap between the Hall effect element and a surface of the rotatable member. The perpendicular distance between the center of rotation and the line along which the proximity sensor is disposed is mathematically determined as a function of the distances between the center of rotation of the rotatable member and the minimum and maximum possible locations of the Hall effect element along with the angular distance between those two dimensions. The sensor provides a gear tooth sensor which is significantly immune to changes in the gap between a Hall effect element and the surface of a rotatable member which is being sensed. The rotatable member is provided with at least one discontinuity in its surface which causes changes in the magnetic field surrounding a Hall effect element and, as a result, provides a signal from the Hall effect element to indicate the proximity of the discontinuity to the Hall effect element.
The Tokura et al patent shows the relative position of a gear tooth sensor to the rotatable target comprising a plurality of gear teeth. The Gagliardi patent shows the internal structure of a gear tooth sensor with a permanent magnet and a magnetically sensitive component disposed internally and proximate a distal end of the sensor. The Stolfus patent shows one particular technique that has been used in order to reduce the sensitivity of a gear tooth sensor to variations in the position of the distal end of the sensor relative to a rotatable target. Accuracy in the position of the sensor relative to the target is very important to the proper operation of the sensor.
It would therefore be significantly beneficial to the field of gear tooth sensing if a means could be provided which allows the gap between a distal end of a gear tooth sensor and a rotatable target to be accurately set without the need of expensive and time consuming techniques and without the need for complex measurements and procedures.